Abstract: Helium is the most commonly used tracer gas for leak detection.
However, it has a number of limitations in many industrial applications. The
molecular weight of hydrogen is only half that of helium, and hydrogen has a much
higher molecular velocity. As a result, hydrogen diffuses rapidly inside test objects,
and dissipates far more quickly than helium. These properties, and the low price of
hydrogen, make it a very attractive alternative.
There are two main reasons why hydrogen is not already more commonly used than
helium as a tracer gas: a) the perception that hydrogen is too dangerous and b) the
availability of suitable hydrogen detectors.
The flammability problem is easily solved by using a readily available standard mix
of 5% hydrogen and 95% nitrogen, which is classified as non-flammable.
The other obstacle was overcome by the introduction of a new type of hydrogen
detector, based on microelectronic hydrogen sensors. These have high sensitivity
and selectivity to hydrogen. Moreover, they are cost-effective and sufficiently robust
for industrial applications, allowing leaks of 5x10-7 mbarl/s to be detected using the
5% hydrogen mix. The use of hydrogen as a tracer gas is now increasing rapidly in
the automotive, chemical, power generation, aerospace and telecommunications
industries, among others.